Heretofore, magnetic recording media of the coated type have been used widely. These recording media are obtained by coating a powdery magnetic material, for example, a magnetic powder such as gamma-Fe.sub.2 O.sub.3, Co-doped gamma-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Berthollide compound of gamma-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 or CrO.sub.2 or a ferromagnetic alloy powder, dispersed in an organic binder such as a vinyl chloride/vinyl acetate copolymer, a styrene/butadiene copolymer, an epoxy resin or a polyurethane resin, on a non-magnetic support and drying the coating. In recent years, with an increased demand for high-density recording, much interest has been aroused in "binderless" magnetic recording media having a thin ferromagnetic metal film formed by vapor deposition techniques such as vapor deposition, sputtering or ion plating, or plating techniques such as electroplating or electroless plating as a magnetic recording layer, and various efforts have been made toward the commercial acceptance of such binderless magnetic recording media.
In the conventional coated magnetic recording media, metal oxides having a saturation magnetization lower than that of ferromagnetic metals are used as a magnetic substance. Hence, thickness reduction required for high-density recording decreases signal outputs, and there is a limit to thickness reduction. Furthermore, the production process is complex, and large accessory devices are required for solvent recovery or the prevention of environmental pollution. The binderless magnetic recording media have the advantage that they can be formed in ultrathin thicknesses for high-density recording because a ferromagnetic metal having a saturation magnetization higher than that of the above oxides can be formed into a thin film in the absence of a non-magnetic substance such as a binder. Moreover, the production process therefor is simple.
Higher coercivity and smaller thickness have been advocated both theoretically and experimentally as one condition for magnetic recording media for high-density recording, and much is expected of the binderless magnetic recording media whose thickness can be easily reduced by one order of magnitude from that of coated type magnetic recording media and which has high saturation flux density.
The vapor deposition technique, in particular, has the great advantage that it does not require a waste liquor treatment as in plating; the production process is simple; and the speed of film deposition can be made high. The oblique vapor deposition method described in, for example U.S. Pat. Nos. 3,342,632 and 3,342,633 is known as a method for producing a magnetic film having a coercive force and a squareness ratio desirable for magnetic recording media by vapor deposition.
Resistance against corrosion and abrasion and running stability are further significant problems with magnetic recording media composed of ferromagnetic metal films. The magnetic recording media are placed in high-speed relative motion with respect to a magnetic head during recording, playback and erasing of magnetic signals. At this time, the magnetic recording media must run smoothly and stably, and should not undergo wear or fracture by contact with the head. The medium is also required to be free from reduction or disappearance of recorded signals by corrosive changes, etc. during storage. The provision of a protective layer has been studied as a means for increasing the durability and weatherability of the magnetic recording media.
Protective layers so far proposed in the art include, for example, those composed of a thin film of an alloy such as an Ni-W alloy (Japanese Patent Application (OPI) No. 43110/1976), an Ni-B alloy (Japanese Patent Application (OPI) No. 2405/1977), an Ni alloy heat-treated to impart higher hardness (Japanese Patent Application (OPI) No. 102605/1976) and an Ni-Cr alloy (Japanese Patent Application (OPI) No. 73108/1978), those composed of an oxide or carbide (Japanese Patent Application (OPI) No. 104602/1975), an oxide magnetic substance provided thereon an alpha-Fe.sub.2 O.sub.3 -type thin film as a protective layer (Japanese Patent Application (OPI) No. 59606/1976), an Si-Si oxide (Japanese Patent Application (OPI) No. 127203/1977), an Si-Si oxide placed on a magnetic layer with Cr therebetween (Japanese Patent Application (OPI) No. 127204/1977), a silicon nitride compound (Japanese Patent Application (OPI) No. 73931/1980), or an La boride layer (Japanese Patent Application (OPI) No. 11626/1981), those composed of an organic material such as a monomolecular layer of a saturated fatty acid (U.S. Pat. No. 4,087,582), and a lubricating liquid layer containing an antioxidant (Japanese Patent Application (OPI) No. 20805/1976). (The term "OPI" as used herein means a "published unexamined Japanese Patent application".)
It is known that the provision of a protective layer composed of a film of a nonmagnetic metal such as Cr, Ti, Sn, Al, Cu, Zr or Ta, by using a sputtering method (Kinzoku Hyomen Gijutsu (Metal Surface Technology), 36(6), 62ff. (1983)), an ion plating method (U.S. patent application Ser. No. 596,789 filed on Apr. 4, 1984), etc., increased weatherability. Such a protective layer, however, does not have sufficient durability, and still requires improvement for commercial acceptance.